This invention relates to a color filter determining apparatus with indication of the color components to be balanced.
The determination of the color filtration of the copying light necessary for the copying of color negatives is frequently accomplished by color filter-determining apparatus whose measuring principle is based upon measurement of the reciprocal proportions of the primary color constituents in the color negative to be copied, -- photoelectric transducers, usually photoconductive cells, being employed as the measuring receivers, forming a branch of a measuring bridge or of a compensating circuit.
In the processing of a series of color negatives, there usually do not occur any excessive changes in the filtration, necessary for color cast-free copying, of one color negative to the next, so that often only the filter values of a single primary color need be changed. For an acceleration of the filter-determining process, it is therefore advantages if the color filter-determining apparatus contains an indicator which shows which of the three primary color components has to be balanced.
German Pat. No. 1,472,610 describes a color filter-determining apparatus wherein three spectrally differently sensitive photoconductive cells, inserted into the copying light beam, are connected within a resistance-compensating circuit, wherein the changes in resistance of the photoconductive cells caused by the effects of light are balanced by respective rotary potentiometers, whose positions depict the measure of color filtration necessary for the copying process. By means of a periodically reversing electromagnatic switch, the tap of a rotary potentiometer may be intermittently connected with one of the two zero indicators each, which in turn are in contact with a tap each of the two other rotary potentiometers. The indicating condition of the zero indicators prior to the balancing gives a clear indication of the filter color which must be balanced, since in both switch positions, the relationship of two primary color constituents to each other is indicated.
This known color filter determining apparatus has, however, the drawback that the color balance of the two equalized primary colors (in the example: blue to red or blue to green) cannot be adjusted independently for both color groups, which reduces the operating flexibility of the apparatus. For example, the setting of the potentiometer pertaining to the primary reference color may be such that an equalization can no longer be accomplished by balancing the potentiometer pertaining to another primary color, but instead is possible only by adjusting the first potentiometer pertaining to the primary reference color, which again changes the balance of the other color group which possibly has been already previously equalized. Thus, it may occur that the balancing of both color groups can be attained only after performing several steps.
With such apparatus, the necessity of periodically reversing the electromagnetic switch between the two zero indicators also has a detrimental effect, because it influences to a great degree the cost of manufacture of the apparatus. A further drawback of such known apparatus is that the individual photoconductive cells are energized at different operating points, determined by calibrating resistors, so that their different time constants become noticeable by a jump in brightness.
An object of this invention is to provide a color filter determining apparatus with an indication of the color components to be balanced, which enables an equalization of the two primary color proportions independent of each other and wherein the above-mentioned drawbacks are avoided.